1. Field of the Invention
The present invention relates to optical fibers (lightguides) which are used in remote spectroscopy applications and in medical applications. These applications require the transmission of either ultra-violet (UV) laser energy for performing surgical procedures or lower energy, UV light for irradiating or illuminating a surface.
2. Information Disclosure Statement
A growing number of applications in spectroscopy and medicine are being designed to isolate the spectrometer or the medical laser/light source from the actual site being viewed or operated on. Optical fibers play a critical part in making such applications possible. In these applications, a need exists to have good, stable transmission in the wavelength region below 325 nm as well as throughout the visible near-IR (infra-red) region of the spectrum. Although spectroscopic applications employ low energy irradiation sources for extended periods of time and the medical applications--especially surgical applications--employ high energy sources for short durations, both applications need stable (i.e. low UV sensitive) optical fibers to function properly. Small drifts in the transmission properties over time affect the output in spectroscopic applications perhaps greater than absorbing defects affect laser light in surgical procedures.
Generally speaking, optical fibers with pure silica cores are the most resilient to irradiation damage if they have a minimum of latent defect centers. Similar to an optical fiber's exposure to ionizing radiation (see, e.g., D. L. Griscom, Overview of Radiation Effects in Fiber Optics, 541 SPIE Proc. 70-88 (1985)), increased exposure to UV radiation increases a fiber's transmission loss (see, e.g., U. Grzesik et al., Reduction of Photodegradation in Optical Fibers for Excimer Laser Radiation, 1649 SPIE Proc. 80-90 (1992)). The transmission performance of the lightguides degrades particularly strongly and quickly when the fibers are used for the transmission of wavelengths below 280 nm (i.e. the far UV range). Similar problems are observed using pulsed excimer laser transmission at 308 nm. The degradation in transmission is observed with Silica/Silica fibers, which usually have a pure, high OH content silica core with a fluorine doped silica cladding, and with Plastic Clad Silica (PCS) optical fibers. The problem can be solved either by frequently replacing the fibers or by cutting off the end of the fiber which is exposed to the source of the UV radiation (i.e. the end receiving the highest UV exposure per unit time) and re-terminating the fiber.
The defect centers which cause the largest decreases in transmission in the UV region for major absorptions at 163 nm, 248 nm and 210 nm are identified as Si--Si, Si.sub.3.sup.+ and E-centers respectively and are related to oxygen vacancies. The process of fiber drawing increases the vacancy defect population, and it was found that the maximum concentration of the vacancies is in the 8-10 .mu.m layer of the core near the cladding layer. The presence of high OH content in the core mitigates much of these problems, but sacrifices transmission in regions above 820 nm where overtones of the strong OH bond absorption occur. Fortunately, for most UV applications of remote spectroscopy or surgery, moderate to high OH levels in the core pose no major detriment.